1. Field of the Invention
The present invention relates to electrical machines and more particularly to noise reduction designs for electrical machines.
2. Description of the Related Art
In the design of electrical machines, reducing audible noise generated by operation of the machine can be important. One field in which noise reduction is particularly important is in the design of alternators for motor vehicles.
An important operating point at which it is very important for an alternator operate quietly is at engine idle. There, because the vehicle is otherwise relatively quiet due to low engine speed and the lack of wind and road noise, alternator noise can be particularly annoying to the occupants of the vehicle.
The audible noise produced by operation of an automotive claw pole alternator is partly electromagnetic in origin. In particular, a third-harmonic field can produce significant noise. In a common 12-pole alternator, this third harmonic field is a 36-pole field. The third harmonic field is due to the interaction of the magnetomotive force produced by the stator currents with the permanence of the air gap between the rotor and the stator.
Electrical machine designs which can reduce the audible noise generated by electromagnetic fields in the machine will provide significant advantages over the prior art. Particularly in the automotive environment, the advantage will be in the form of increased customer satisfaction.